The present invention relates to a developing device for an electrophotographic copier and others and, more particularly, to a magnetic brush type multiple color developing device for twin-color, multi-color or full-color development which includes a plurality of developing units each having a developing roller or a magnetic brush type developing device which effects development with a wide range of tones by using toner of a single color.
In a multiple color developing device, for example, a plurality of developing units each storing toner of a different color are arranged in close proximity to a photoconductive element which serves as a latent image carrier. Development is repeated a plurality of times with the toner of different colors to produce a final color image. Among such developing devices, one which uses a magnetic brush includes a developer carrier which is installed in each developing unit for forming the magnetic brush. The developer carrier is usually implemented with a developing roller which is made up of a cylindrical sleeve, and a magnetic body received in the sleeve for forming the magnetic brush. At least one of the sleeve and magnetic body is rotated to transport a magnetic brush between a toner supply region and a developing region. The developing units are selectively operated on a color basis. Specifically, while any one of the developing units is in operation, the others are maintained inoperative and, thereby, prevented from supplying toner to a latent image which is provided on the photoconductive drum.
However, it sometimes occurs that even when a developing unit which does not join in development is held inoperative, toner deposited on the developing roller of that developing unit makes contact with a latent image to be transferred to the latter. The disturbs the reproduction of colors resulting in mixture of colors.
Various schemes have heretofore been proposed to solve the above-stated problem. Those schemes include one which causes only the developer carrier to move in a reciprocal motion (Japanese Laid-Open Patent Publication (Kokai) No. 50-151532/1975 etc.), one which allows a blade to move into and out of contact with the developer carrier (Japanese Laid-Open Patent Publication (Kokai) No. 50-151530/1975 etc.), one which selectively rotates the developing sleeve in opposite directions (Japanese Laid-Open Patent Publication (Kokai) No. 50-93438/1975 etc.), one which angularly moves, or rotates, the magnets which are installed in the developer carrier (Japanese Laid-Open Patent Publication (Kokai) No. 52-105832/1977 etc.), and one which includes a shutter device located between the developer carrier and the latent image carrier (Japanese Laid-Open Utility Model Publication (Kokai) No. 52-152742/1977). All such prior art schemes, however, lack simplicity and sureness.
On the other hand, Japanese Laid-Open Utility Model Publication No. 60-39053/1985 discloses a developing device in which a shield member is located in a space between a cylindrical developing sleeve which neighbors a latent image carrier in a form of drum and a magnetic body (magnet roll) which is received in the sleeve. The shield member is rotatable to change the magnetic force distribution on the sleeve by shielding the magnetic field which is developed by the magnet roll, whereby the timing of developer supply to the sleeve is controlled. What should be noted in this prior art developing device is that the shield member is rotated along the above-stated space to shield even the magnetic field which is developed by a main pole magnet of the magnet roll that faces a photoconductive element. In such a construction, a sufficient space should be available between the shield member and the magnetic roll and between the sleeve and the shield member, not to speak of the space between the sleeve and the magnet roll.
On the other hand, the wall of the sleeve has to be thinned as far as possible so that sufficient magnetic forces may be developed on the sleeve. For example, in the above-described prior art device, when the wall thickness of the sleeve is as thin as 0.8 to 1 millimeter, the spacing available between the inside diameter of the sleeve and the outside diameter of the magnet roll is not more than 2 to 2.5 millimeters at maximum. Hence, the device has inevitably be configured such that the distance between the shield member and the inside diameter of the sleeve is about 0.5 millimeter, the thickness of the shield member is about 1 millimeter, and the distance between the shield member and the outside diameter of the magnet roll is about 1 millimeter.
Therefore, a problem with this type of developing device is that when the shield member is implemented with an extensively usable cold-rolled steel sheet, it is deformed at an intermediate point of its lengthwise dimension (about 350 millimeters) tending to make contact with the magnet roll. The magnetic force exerted by the magnet roll to deform the shield plate so is generally inversely proportional to the distance between the shield member and the magnet roll. It follows that in a developing device in which a sufficient spacing is unavailable between the shield member and the magnet roll due to its inherent structure, the shield plate is deformed by the magnetic force into contact with the magnet roll resulting that the load in the event of rotating the shield plate is increased to damage a support for the shield plate or disturb the rotation of the shield plate.
Further, in this prior art developing device, the sleeve, magnet roll and shield member are each cantilevered by using a great number of support members. Tolerances of such numerous support members would accumulate to lower the accuracy of a predetermined gap which is defined between the photoconductive element and the developing sleeve, while causing the sleeve to oscillate due to the instability of an axis.
In addition, in a prior art developing device of the type described, the end portion of the shield member and a shield member drive portion for rotating the shield member are integrated by a relatively thick flange (a sufficiently thick flange is indescribable for cantilever type support). For this reason, the width of the sleeve is far greater than that of the magnet roll so that it fails to be matched with those of the other structural elements such as the photoconductive element. This obstructs the common application of drive shafts, brackets and other parts and, thereby, invites an increase in cost.